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Astronomical Observatory: Cool Images

Astr212 Galaxy Projects, Spring 2007

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M82 (Cigar Galaxy), John VanderHeide

M82

On December 31, 1774, Johann Elert Bode discovered two “nebulae” located near each other in the northern sky.  These nebulae were later cataloged as M81 and M82 by the comet hunter Charles Messier  who was keeping a record of objects on the sky that looked like comets but weren’t.  Little did either of these astronomers know that these two objects were not nebulae, but in fact whole galaxies, similar to our own in size and shape.  M82, seen above, is the smaller of the pair; it is located 12 million light years from earth and is 4 Kpc across.  It is classified as an I0 Starburst galaxy.  This means that the structure of the galaxy is not easily determined due to a recent interaction with one of its neighbors that is causing massive amounts of star formation.  That companion is the more massive M81.  M82 used to be a typical spiral galaxy slightly smaller than our own, but through gravitational interactions with M81 and the other galaxies in its group it has lost most of its spiral structure and it has started forming a large number of new stars.  These stars can be seen in the image as the bluish coloration at the ends of the galaxy.  The center of the galaxy appears redder than the rest because of the large amounts of dust that is present there.  The dust causes the blue light from that region to diffract (bend out of the line of sight) while allowing the redder light (in this case the infrared light) to pass through unhindered.  This is the same phenomenon that causes the daytime sky to be blue and sunrises and sunsets to appear reddish.  Just to the left of the bright central core you can see an exceptionally dense region of dust that obscures most of the light coming from behind it;  this can be seen as a dark lane running across the galaxy.

Multi-wavelength Comparison:

M81 Group

This picture is courtesy of the NRAO http://www.nrao.edu/imagegallery/php/level3.php?id=116

This is a picture of the entire M81 group. The left image is the visual part of the spectrum while the right image of the galaxies was taken in the radio part of the spectrum. Here the intensity of the radiation is coded by the false color, the brighter regions appear red in this image and the dimmer regions are colored blue.  This particular spectral line of radiation with a wavelength of 21cm is dominated by an emission line characteristic of neutral hydrogen gas.  Therefore studying this line allows us to track how the gas in this group of galaxies is being affected by their interaction.  M81 is the large spiral near the center of the image, M82 lies above M81 in this image and NGC 3077 is the small galaxy off to the left.   The long tails of hydrogen that can be seen connecting the galaxies are caused by tidal interactions that happen as a result of the galaxies being so near to each other, these same interactions are causing the rapid star formation in M82. Tidal interactions are caused when two objects interact gravitationally. Because the strength of the gravitational interaction depends on the distance between two objects the close sides of the two objects feel the pull stronger than the far sides of the objects. This difference in the gravitational force is what causes the tidal tails that you see.

M82 HST, Spitzer, Chandra

This image is courtesy of NASA http://chandra.harvard.edu/photo/2006/m82/index.html

This is a composite picture of M82 taken with three of NASA’s space telescopes.  This is a false color image where different colors indicate different regions of the electromagnetic spectrum.  The Hubble Space Telescope provided information in the optical range of the spectrum.  This data appears yellow, green, and orange in this image. The Spitzer Space Telescope took infrared data for this picture, this data is coded red. The final set of data that is included is that from the Chandra X-Ray Observatory, which appears blue in this image.   The yellow and green images show us wavelengths that stars emit.  This light is concentrated in the disk of the galaxy.  The orange data is rather hard to distinguish from the red data in this image because they are both highlighting the violent outflow of dust and gas from the galaxy.  This outflow is caused by the rapid star formation.  As vary large stars form they produce immense stellar winds that push gas and dust out of the galaxy.  In M82 the rate of star formation is so high that we can actually see the gas and dust that is being pushed out by those stellar winds.  The dust is emitting in the infrared and optical because it is being heated up by the winds that are pushing it out of the galaxy. The X-rays have far more energy than do optical light or infrared light.  In this image the X-rays are being emitted by the extremely hot gas that actually makes up the stellar winds which are pushing the other dust out of the galaxy.  This image is a mirror image of the one that I took, to compare the two you would need to flip it horizontally.

Light Profile: 

Considering the there are many different types of galaxies ranging from highly organized grand design spirals to randomly arranged ellipticals, and that they are all found at such varying distances; it is often difficult to characterize a galaxy so that it can be compared to other galaxies.  One way that astronomers have taken to comparing galaxies is by comparing their light profile.  A light profile is the brightness of the light coming from the galaxy at each point along a line that crosses the galaxy.  By doing this kind of analysis we are able to have a better understanding of the structure of the galaxy.

The best way to model an irregular galaxy link M82 is to consider it to be a spiral galaxy that is highly disturbed.  Through many observations astronomers have determined that the light from the disk of a spiral galaxy is best modeled by an exponential function.  To determine what exponential equation modeled the brightness of M82 we plotted the natural log of the brightness verses the distance from the center in Kpc.  This data was used to find an exponential fit that gave us our model. The actual brightness of M82 and the brightness predicted by the model were then graphed together in these next two graphs. Fits were done for both the major and minor axis of M82.

M82 Light Profile - Major Axis

M82 Light Profile Minor Axis

This fitting process provided us with a characteristic radius that tells us about the distribution of the light across the galaxy. The major axis radius was 0.3+/- 0.003Kpc and the minor axis radius was 0.2+/- 0.002Kpc. These scale lengths are a bit smaller that the typical scale length of a galaxy, but that is expected considering how small M82 is. The things of interest to note about these light profiles is that the major axis light profile shows how extreme the dust extinction is in the center of M82. Yet the minor axis does not show similar extinction. The difficulty of doing this kind of fit on M82 is that this system is so disturbed from normal that the function fit in outer part of the galaxy does not apply to the central region.

Extinction:

One of M82's most defining features is the dust lane that crosses the galaxy near the central bulge. We thought it would be interesting to estimate how much that dust lane was obscuring the light from behind. To do this we needed to figure out how bright that part of the galaxy would be if there was no dust lane. To do this we repeated the light profile calculations with the data taken in the B and V filters, here are the graphs.

M82 Light Profile - B filter

M82 Light Profile - V filter

The dust lane is found about 0.1Kpc to the left of the center of the galaxy. To calculate the extinction we found the ratio of predicted brightness to observed brightness. This value gave us the decrease in brightness due to the dust in units of magnitudes. The dust dimmed the light by 2.5 mag in B and 2.2 mag in V. From there we compared how the extinction in B was different from the extinction in V by using the formula R= ΔMv/(ΔMb-ΔMv). R is the ratio of the extinction to the reddening. The value for typical dust grains is 3.1, here we found that R=6.4, which is significantly higher.

Data Reduction:

All of our data were taken on the Calvin-Rehoboth Observatory 16in. Optical Guidance Systems telescope. The Individual images that were taken were dark subtracted and bias and flat corrected. The calibrated images were then aligned and median combined by color to make a best image in each color(Clear, Blue, Visual, and Infrared). To make the final image the four images were combined in Maxim. The B image was set to be blue, the V image was set to be green, and the I image was set to be red, with the C image serving as the luminosity standard for the combined image. This yielded a false color image because what appears red is actually infrared photons. Once an appropriate color balance was achieved(B 500%, V 75%,I 65%) the image had its color saturation increased to enhance the structure. At this point the fits file was converted to a jpeg and the brightness scale was changed from a linear rendering to a gamma stretched rendering, this further enhanced the fine detail on the edges of the galaxy.

References:
"Messier 82" <http://seds.lpl.arizona.edu/Messier/m/m082.html>

"Tidal Interactions in M81 Group" <http://www.nrao.edu/imagegallery/php/level3.php?id=116>

"M82: Images from Space Telescopes Produce Stunning View of Starburst Galaxy" <http://chandra.harvard.edu/photo/2006/m82/index.html>

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Kutner, Marc L. Astronomy: A Physical Perspective. 2nd ed. Cambridge, United Kingdom: Cambridge University Press, 2003. Ch.14.2.3 p 240-242

Right Ascension (J2000) 09:55.8
Declination (J2000) +69:41:00
Filters used blue(B), green(V), infrared(I), & clear(C)
Exposure time per filter 5x60 seconds in C, 300 seconds in BVI
Date observed

Febuary 22 , 2007 (B,V,I)
March 12, 2007 (B)
March 15, 2007 (C,V,I)

 

 

 

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